Plant root growth and the marginal value theorem

Abstract

All organisms must find and consume resources to live, and the strategies an organism uses when foraging can have significant impacts on their fitness. Models assuming optimality in foraging behavior, and which quantitatively account for the costs, benefits, and biological constraints of foraging, are common in the animal literature. Plant ecologists on the other hand have rarely adopted an explicit framework of optimality with respect to plant root foraging. Here, we show with a simple experiment that the marginal value theorem (MVT), one of the most classic models of animal foraging behavior, can provide novel insights into the root foraging behavior of plants. We also discuss existing data in the literature, which has not usually been linked to MVT to provide further support for the benefits of an optimal foraging framework for plants. As predicted by MVT, plants invest more time and effort into highly enriched patches than they do to low-enriched patches. On the basis of this congruency, and the recent calls for new directions in the plant foraging literature, we suggest plant ecologists should work toward a more explicit treatment of the idea of optimality in studies of plant root foraging. Such an approach is advantageous because it forces a quantitative treatment of the assumptions being made and the constraints on the system. While we believe significant insight can be gained from the use of preexisting models of animal foraging, ultimately plant ecologists will have to develop taxa-specific models that account for the unique biology of plants.

Fig. 1.

Schematic representation of experimental design. Regions of differing soil quality are indicated by regions of darker shading. High-quality soil contained 66% manure mixed with background soil (vol/vol), low contained 25%, poor 4%, and background soil 0%. Transparent plastic tubes spanned the length of each box so that roots traveling away from the shoot could be visualized through the use of a minirhizotron camera inserted into the tubes. Minirhizotron tubes below the soil in the schematic are indicated by dashed lines and are shown for the Hom treatment only. Distance traveled by roots searching for nutrients in the soil was measured as the distance from the base of the shoot to the farthest visible root either toward patches, or away from patches. Schematic is not to scale.

Fig. 2.

Summary of the optimal patch use behavior of plants according to the hypotheses generated by marginal value thereom. (A) Mean raw distance traveled by plant roots through the soil across all treatments without accounting for shoot size, after 36 days of growth. The location of the first patch is denoted by dashed horizontal lines. (B) Mean distance traveled by roots standardized by plant size, all treatments after 36 days of growth. This accounts for differences in distance traveled by roots that are related only to plant size. (C) Mean soil exploration measured as biomass of roots inside the boundaries of the first patch only (toward) and the equivalent location on the opposite side of the plant (away) for each treatment. Letters above bars represent the differences in mean proliferation designated by the least square means post hoc comparison in SAS. Data are after 48 days of growth. (D) Mean total biomass of plants grown in each soil treatment after 48 days of growth. Changes in biomass reflect differences in nutrient uptake among plants. We did not directly measure plant fitness. Letters above the means indicate statistical differences detected by a Tukey's test. Error bars in all panels are 1 SE.